Vibrating fluid sensors, such as Coriolis mass flow meters and vibrating densitometers typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the fluid in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness and damping characteristics of the containing conduit and the material contained therein.
A typical vibrating fluid meter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes, including for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Two such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. The two pickoff sensors are connected to electronic instrumentation by cabling, such as by two independent pairs of wires. The instrumentation receives signals from the two pickoff sensors and processes the signals in order to derive a mass flow rate measurement.
Vibrating fluid meters offer high accuracy for single component flows. However, when a vibrating fluid meter is used to measure fluids including entrained gas, gas including entrained liquid droplets, or other types of fluids including both compressible and incompressible components, the accuracy of the meter can be significantly degraded. Entrained gas is commonly present as bubbles in the flow material. One problem caused by gas bubbles is decoupling. Small bubbles typically move with the liquid flow material as the flow meter is vibrated. However, larger bubbles do not move with the liquid during vibration of the flow tube. Instead, the bubbles can be decoupled from the liquid and can move independently of the liquid. Consequently, the liquid can flow around the bubbles. This adversely affects the vibrational response of the fluid meter.
The size of the entrained gas bubbles can vary, depending on the fluid velocity, viscosity, surface tension, and other parameters. The extent of the decrease in performance is not only related to how much total gas is present, but also to the size of the individual gas bubbles in the flow. The size of the bubbles affects the accuracy of the measurement. Larger bubbles occupy more volume and decouple to a greater extent, leading to greater error in measurements of the flow material. Due to the compressibility of a gas, the bubbles can change in gas amount yet may not necessarily change in size. Conversely, if the pressure changes, the bubble size can correspondingly change, expanding as the pressure drops or shrinking as the pressure increases. This can also cause variations in the natural or resonant frequency of the flow meter.
Vibrating fluid meters are used to perform mass flow rate and density measurements for a wide variety of fluid flows. One area in which Coriolis flow meters can be used is in the metering of oil and gas wells. The product of such wells can comprise a multicomponent fluid, including the oil or gas, but also including other components, such as water and air, for example. It is highly desirable that the resulting metering be as accurate as possible, even for such multicomponent flows. Further, in such situations, a user often wants to know not only the overall flow rate and density of the fluid, but other fluid characteristics, such as the density of the liquid phase and the flow rate of the individual components of the multicomponent flow. Often, the Coriolis flow meter will measure only an overall flow rate and density of the fluid. In the case of two liquid components of known density, it is possible in prior art flow meters to determine the individual component fractions and flow rates. The flow meter electronics currently on the market make the assumption that the fluid flow only contains oil and water and use equations (1) and (2) to determine the amount of each component. This algorithm is known in the oil and gas industry as a Net Oil Computer.φo+φw=1  (1)ρmeasured=φoρo+φwρw  (2)
Where:
φo is the volume fraction of oil;
φw is the volume fraction of water;
ρmeasured is the fluid density measured by the vibrating fluid meter;
ρo is the oil density; and
ρw is the water density.
Using equations (1) and (2), if the water and oil densities are either known or assumed, then the volume fractions for the oil and water can be determined. With the volume fractions determined, the flow rate of the individual components can be determined. It is important to note that the measured density in equation (2) is actually slightly inaccurate due to decoupling between the two different components. However, because the density of water and oil are similar, the decoupling is very small and measurements are generally accurate enough.
However, when a system only uses equations (1) and (2), and if entrained gas is present, the resulting lower overall fluid density is incorrectly interpreted to be caused by higher oil volume fraction and thus, the meter electronics outputs a higher oil flow rate and overall amount of oil in the stream. In many real-world applications, the fluid may contain some gas, which may drastically reduce the measurement accuracy of the Net Oil Computer. Therefore, the fluid may not contain as much oil as output by the fluid meter. This can be problematic as a user may think the oil well is still producing a satisfactory amount of oil while the well is actually only producing water and gas. The presence of gas within the system results in equations (1) and (2) transforming into equations (3) and (4).φo+φw+φg=1  (3)ρmeasured=φoρo+φwρw+φgρg  (4)
Where:
φg is the volume fraction of the gas; and
ρg is the gas density.
As can be seen, equations (3) and (4) result in two equations, but three unknowns (the three volume fractions), which do not have a unique solution.
Another area where vibrating fluid meters are used is in the food and beverage industry. For example, in the dairy industry, users may want to know the density of the milk being delivered for various processing and quality reasons. However, often, the flowing milk includes entrained air bubbles. Therefore, for a given density provided by the vibrating fluid meter, a user cannot be sure of the density of the milk as the measured density is affected by the lower density of the air. Additionally, the volume fractions of the milk and air are unknown.
Although the problems outlined above have involved mainly liquids with entrained gas, it should be appreciated that similar problems exist with multicomponent fluids containing one or more compressible liquids mixed with one or more incompressible liquids. By “compressible” it is meant that the density of the component changes by a threshold amount within the operating conditions experienced within the system of interest. These multicomponent fluids may comprise liquids with entrained gas, two or more liquids (with at least one comprising a compressible liquid), or a gas with entrained liquid droplets.
There remains a need in the art for a vibratory fluid meter that can accurately measure flow characteristics of a multicomponent fluid with one or more incompressible fluids and one or more compressible fluids.